Electroactive polymers (EAPs) are polymers that exhibit a change in size or shape when stimulated by an electric field or reversibly generate energy when motioned. Typically, an EAP is able to undergo a major deformation while sustaining large forces.
The development of elastomeric materials with high dielectric permittivity has attracted increased interest over the last years due to their use in e.g. dielectric electroactive polymers (DEAP's).
Dielectric electroactive polymers are materials in which actuation is caused by electrostatic forces on an elastomeric film sandwiched between two electrodes which squeeze the elastomer upon application of an electric field. When an electric voltage is applied, an electrostatic pressure is exerted on the film, reducing its thickness and expanding its area due to the applied electric field. Examples of EAP's are dielectric elastomers. Dielectric electroactive polymers are used e.g. as actuators as so-called “artificial muscles” and as generators in energy-harvesting.
However, a drawback of DEAP's for a wide range of applications is the high operation voltage, which tends to be several thousand volts when actuation strains higher than 2-3% are wanted. The operation voltage can be reduced by reducing the thickness of the elastomer film, decreasing the mechanical stiffness of the material or increasing the relative dielectric permittivity thereof. A reduction of the thickness to less than 5 μm seems, however, not possible for mass-produced films (Matysek et al., in Proc. SPIE-EAPAD, San Diego, p. 76420D (2010)). A reduction of the stiffness has been shown in a tri-block copolymer using block specific oil as plasticizer (Shankar et al., Macromol. Rapid Comm. 28, 1142-1147 (2007)). An increase in relative dielectric permittivity (∈r) of a material can lead to high electrical energy density with lowered operating voltages. Permittivity enhancement and stiffness reduction was shown by blending a conducting poly(3-hexylthiophene) into a commercially available polydimethylsiloxane (PDMS) elastomer (Carpi et al., Adv. Funct. Mater. 18, 235-241 (2008)). Another approach to enhance dielectric permittivity is attaching or grafting small molecules with high permanent dipoles into an elastomer matrix (Kussmaul et al., Adv. Funct. Mater., 2011, 21, 4589-4594).
However, the prior art dielectric electroactive polymers exhibit a relative dielectric permittivity (∈r) of only about 5-20 at 0.1 Hz and it is envisaged that the energy density of DEAP's should be substantially higher in order to be commercially interesting. Thus the dielectric permittivity seems to be an important tuning parameter for obtaining DEAP's with a high energy density.